1. Field of the Invention
The present invention relates to an optical retardation plate using a dielectric medium, and a method of manufacturing the same.
2. Related Background Art
Heretofore, an optical retardation plate is manufactured by polishing a crystalline plate made of a quartz crystal. In this case, a thickness of the optical retardation plate is adjusted so that a phase retardation between ordinary light and extraordinary light becomes an (N+¼) wavelength (N: integral number) in a ¼ retardation plate, an (N+½) wavelength in a ½ retardation plate, and an N wavelength in a full retardation plate.
In addition to such a method utilizing the crystal polishing, a method using a grating utilizing structure double refraction based on a structure of dielectric is also proposed. Proposal and experiments on a retardation plate using this grating is described in an article written by D. C. Flanders: Applied Physics Letters, Vol. 42, No. 6 (Mar. 15, 1983), pp. 492 to 494.
The optical retardation plate using a grating utilizes a fact that a refractive index na in a direction parallel to an extension direction of trenches of the grating and a refractive index nb in a direction intersecting perpendicularly the extension direction of the trenches of the grading are different from each other in an area where a wavelength λ is sufficiently smaller than a pitch d. According to the above article, when the grating has a rectangular shape, the refractive index na and the refractive index nb are given by Equations (1) and (2), respectively:na={n12+n22(1−q)}1/2  (1)nb={(1/n1)2q+(1/n2)2(1−q)}−1/2  (2)
where n1 is a refractive index of a first medium, n2 is a refractive index of a second medium, and q is a ratio which the first medium occupies for one period of the grating and which is in a range of 0≦q≦1. Then, a magnitude Δn of double refraction is given by Equation (3):Δn=|na−nb|  (3)
In addition, when a depth of each trench in the grating is assigned D, a phase retardation ΔΦ which light made incident to the grating having the double refraction with the magnitude Δn undergoes is given by Equation (4):ΔΦ[rad]=(2πD/λ)Δn  (4)
It is understood from Equation (4) that in order to obtain a large phase retardation ΔΦ, the depth D of each trench has to be increased or the magnitude Δn of the double refraction has to be increased. This relationship is established not only when the grating shape is rectangular, but also when the grating shape is a sine wave-like shape, a chopping wave-like shape, or the like.
In order to concretely manufacture a retardation plate using the grating based on the above-mentioned principles, the following two methods are mainly, readily considered. A first method is a method in which a grating is formed in a photoresist film by utilizing an interference exposure method, a mold is manufactured from the resultant grating, and the grating is transferred to a thermoplastic resin through the mold by utilizing a hot press method or an injection molding method, or is transferred to a photocuring resin.
A second method is a method in which a grating made of a photoresist film is formed on a dielectric substrate similarly to the first method, and the dielectric substrate is selectively etched away by utilizing an ion etching method or a reactive ion etching method using the photoresist film as an etching mask to obtain a grating on a surface of the dielectric substrate.
When such a grating is manufactured by utilizing the first method, since a substantial contact surface area between a medium and an electroformed mold remarkably increases, a tensile shear force when the medium is peeled from a mold surface becomes large. For this reason, there is encountered a problem that the cured medium is peeled off from a substrate during the peeling to be left on the mold surface, and hence the transfer of the grating becomes difficult.
In addition, in the case of the second method, since a time required for the etching extends over several hours and hence the photoresist mask able to withstand the etching is necessarily thickened, the formation of the photoresist mask is difficult.
In addition, in a case as well where a grating formed on a photoresist film is transferred to a material having high etching resistances e.g., a chromium (Cr) film, and an etching is carried out using the chromium film as an etching mask, progress of the etching is blocked since along with an increase in depth of each trench of the grating, a dielectric medium which is etched once is restucked to a surface of a substrate, and the number of particles such as active radicals, ions or neutral particles which arrive at bottom portions of trenches decreases. Thus, it is difficult to form a grating having a desired shape. Such a problem arises irrespective of a shape of a grating. Also, when a size of a substrate is large, uniformity within a surface during the etching becomes poor, and hence an excellent manufacture yield cannot be obtained.
JP 7-99402 B discloses that a grating is covered with a dielectric medium having a sufficiently large refractive index to make a depth of each trench small in order to cope with those problems. In case of this method as well, however, the dielectric medium is difficult to be formed to a bottom portion of each trench since a width of each trench is small.
FIG. 13 shows a conventional manufacture example in a general film formation state in which no film management is carried out, i.e., FIG. 13 shows a cross section image, observed with a scanning electron microscope (SEM), of a formed film which is obtained by mixedly forming an amorphous film and a crystallized film each made of titanium dioxide (TiO2) on a quartz wafer. In this cross section image, an upper portion having a columnar structure is a crystallized area and a lower portion is an amorphous area.
FIG. 14 shows a cross section image, observed with a SEM, of a grating which is formed by selectively etching away the dielectric medium. It is generally known that a crystallized area and an amorphous area are different in etching rate, and the etching rate of the amorphous area is higher than that of the crystallized area. From this fact, the cross section image of FIG. 14 shows that since side etch for a lower area especially progresses, this lower area is the amorphous area.
When crystallized particles mixedly exist in an amorphous dielectric medium, since the etching rate of portions of the crystallized particles is low, the portions of the crystalline particles are left as shown in FIG. 15 without being etched away. In addition, the crystallized particles function as an etching mask during the etching and hence portions under the crystallized particles are not etched away. Thus, an optical retardation plate using a grating cannot be satisfactorily manufactured.
The phase retardation ΔΦ which the light made incident to the grating undergoes, as shown in Equation (4), is proportional to the depth D of each trench of the grating and the magnitude Δn of the double refraction. Then, a dielectric medium which is much larger in refractive index than a dielectric substrate is formed on the dielectric substrate to form a grating shape, thereby allowing the depth D of each trench of the grating to be made small. However, when the dielectric medium is crystallized or the crystallized particles exist, the etching rate differs depending on lattice directions of a crystal, or the amorphous area and the crystallized area are different in etching rate. Accordingly, the uniform etching becomes difficult to be carried out.